Feedback Principles and Types of Feedback Amplifiers for SS 3

Class: SS 3
Term: First Term
Week: 4
Age: 16 years
Duration: 45 minutes
Subject: Basic Electronics
Curriculum Theme: Feedback Circuits
Previous Lesson: Applications of Amplifiers
Topic: FEEDBACK CIRCUIT
Subject Matter: Principles of feedback and types of feedback amplifiers e.g. positive and negative feedback.

Specific Objectives

By the end of the lesson, pupils/students should be able to:

Cognitive Domain

• Define feedback in an electronic circuit.
• Explain the principles of feedback in amplifiers.
• Differentiate between positive and negative feedback.
• State the effects of negative feedback on amplifier performance.
• Identify applications of positive feedback amplifiers.

Affective Domain

• Appreciate the importance of feedback in electronic circuit design.
• Show interest in understanding complex amplifier configurations.

Psychomotor Domain

• Draw simple block diagrams of feedback amplifiers.
• Interpret diagrams of different feedback amplifier types.

Reference Materials

The following resources were used in planning this lesson:

• 2025 Revised 9 Years Basic Education Curriculum
• Relevant State Unified Scheme of Work
• Basic Electronics textbooks for Senior Secondary School
• The HeadTeacher Scheme of work

Instructional Materials

The teacher will teach this lesson with the aid of:

• Chart of diagrams of feedback amplifiers (series-shunt, shunt-series, etc.)
• Whiteboard and markers
• Projector (optional, for displaying diagrams)

Rationale for the Lesson

Understanding feedback is fundamental in electronics as it is a crucial concept in designing stable and efficient amplifier circuits. This lesson helps students grasp how feedback modifies amplifier characteristics, which is essential for further studies in electronic circuit analysis and design.

Prerequisite/Previous Knowledge

Students should have a basic understanding of amplifiers, their characteristics (gain, input/output impedance), and basic circuit components like resistors, capacitors, and transistors.

Lesson Content/Board Summary

FEEDBACK CIRCUIT

Principles of Feedback

Feedback in an electronic circuit is the process of returning a fraction of the output signal back to the input. This returned signal can either reinforce (positive feedback) or oppose (negative feedback) the original input signal. The main components of a feedback system include:

1. Amplifier: The main circuit whose characteristics are modified.
2. Feedback Network: A circuit that samples the output and returns a fraction of it to the input.
3. Sampling Network: Determines how the output signal is picked up (voltage or current).
4. Mixing Network: Determines how the feedback signal is combined with the input signal (series or shunt).

The overall gain of a feedback amplifier is given by the formula:

( A_f = frac{A}{1 + beta A} )

Where:

1. (A) = Open-loop gain of the amplifier (gain without feedback)
2. (beta) = Feedback factor (fraction of output fed back)
3. (A_f) = Closed-loop gain of the feedback amplifier (gain with feedback)

Types of Feedback Amplifiers

Feedback amplifiers are broadly classified into two main types based on the phase relationship between the feedback signal and the input signal:

1. Positive Feedback Amplifiers
2. Negative Feedback Amplifiers

Positive Feedback

Positive feedback occurs when the feedback signal is in phase with the input signal. This means the feedback signal adds to the input signal, increasing the effective input to the amplifier. The term ((1 – beta A)) is used in the denominator of the gain formula for positive feedback, becoming ( A_f = frac{A}{1 – beta A} ).

Effects of Positive Feedback:

1. Increases the overall gain of the amplifier significantly.
2. Can lead to instability and oscillation if the loop gain ((beta A)) is equal to or greater than 1.
3. Reduces bandwidth.

Applications of Positive Feedback:

1. Used in oscillator circuits to generate continuous AC signals.
2. Used in regenerative receivers.
3. Used in Schmitt triggers for waveform shaping.

Negative Feedback

Negative feedback occurs when the feedback signal is 180 degrees out of phase with the input signal. This means the feedback signal subtracts from the input signal, reducing the effective input to the amplifier. The term ((1 + beta A)) is used in the denominator of the gain formula for negative feedback.

Effects of Negative Feedback:

1. Reduces the overall gain of the amplifier.
2. Increases stability, making the amplifier less sensitive to variations in component values and temperature.
3. Increases bandwidth, allowing the amplifier to operate over a wider range of frequencies.
4. Reduces distortion in the output signal.
5. Reduces noise generated within the amplifier.
6. Modifies input and output impedances (can increase or decrease depending on the configuration).

Applications of Negative Feedback:

1. Widely used in almost all linear amplifiers (audio amplifiers, operational amplifiers).
2. Used in voltage regulators to maintain a stable output voltage.
3. Used in control systems for precise control.

Teaching Methods/Instructional Techniques

Discussion, Explanation, Question and Answer, Demonstration, Guided Practice

Instructional Procedures

Step 1: Introduction

Time: 5 minutes

Teaching Skill: Engaging/Recalling

Teacher’s Activity: The teacher greets the students and asks them what they understand by the term “feedback” in general life (e.g., feedback from a teacher, feedback from a microphone). The teacher then links this to electronic circuits.

Pupils’ Activity: Students respond to the teacher’s questions and share their understanding of feedback.

Learning Point: General concept of feedback

Step 2: Explanation of Feedback Principles

Time: 8 minutes

Teaching Skill: Explaining/Illustrating

Teacher’s Activity: The teacher explains the concept of feedback in electronic circuits, defining it as returning a portion of the output to the input. The teacher uses a simple block diagram on the chart to illustrate the feedback loop, highlighting the amplifier and feedback network.

Pupils’ Activity: Students listen attentively, observe the diagrams, and ask questions for clarification.

Learning Point: Definition of feedback

Step 3: Introducing Types of Feedback

Time: 7 minutes

Teaching Skill: Classifying/Differentiating

Teacher’s Activity: The teacher introduces the two main types of feedback: positive and negative feedback. The teacher explains that the difference lies in the phase relationship between the feedback signal and the input signal.

Pupils’ Activity: Students listen and note the two types of feedback.

Learning Point: Types of feedback

Step 4: Detailed Explanation of Positive Feedback

Time: 6 minutes

Teaching Skill: Explaining/Illustrating

Teacher’s Activity: The teacher explains positive feedback, its effects on amplifier gain and stability, and provides examples of where it is applied, using the chart of diagrams where appropriate to show oscillator circuits.

Pupils’ Activity: Students listen, observe the diagrams, and take brief notes on positive feedback and its applications.

Learning Point: Positive feedback characteristics

Step 5: Detailed Explanation of Negative Feedback

Time: 8 minutes

Teaching Skill: Explaining/Comparing

Teacher’s Activity: The teacher explains negative feedback, detailing its various beneficial effects on amplifier performance such as increased stability, reduced distortion, and increased bandwidth. The teacher also highlights its widespread applications in linear amplifiers.

Pupils’ Activity: Students listen, compare the effects with positive feedback, and note down the advantages and applications of negative feedback.

Learning Point: Negative feedback characteristics

Step 6: Discussion and Comparison

Time: 4 minutes

Teaching Skill: Facilitating Discussion

Teacher’s Activity: The teacher facilitates a brief discussion, asking students to orally compare and contrast positive and negative feedback based on their understanding of the effects and applications.

Pupils’ Activity: Students actively participate in the discussion, sharing their comparisons.

Learning Point: Distinguishing feedback types

Step 7: Evaluation/Review

Time: 5 minutes

Teaching Skill: Questioning/Assessment

Teacher’s Activity: The teacher evaluates the learning by asking the following questions:

1. What is feedback in an electronic circuit?
2. State two effects of negative feedback on an amplifier.
3. When is positive feedback typically used?
4. Differentiate between positive and negative feedback based on their phase relationship.

Pupils’ Activity: Pupils answer orally and in writing.

Learning Point: Understanding feedback concepts

Step 8: Note-Taking

Time: 4 minutes

Teaching Skill: Guided Writing

Teacher’s Activity: The teacher guides pupils/students to copy the essential Board Summary notes on feedback principles and types of feedback amplifiers into their notebooks.

Pupils’ Activity: Pupils/students copy the notes carefully into their notebooks.

Learning Point: Recording lesson information

Step 9: Conclusion

Time: 2 minutes

Teaching Skill: Summarising

Teacher’s Activity: The teacher briefly recaps the main points of the lesson, emphasizing the importance of feedback in amplifier design and its role in stability and performance. The teacher encourages students to review their notes.

Pupils’ Activity: Students listen and prepare for the next lesson.

Learning Point: Lesson consolidation

Continuous Assessment/Further Study

Type: Homework

Instruction: Answer the following questions in your notebook:

1. Explain the meaning of feedback in an amplifier circuit.
2. List three advantages of using negative feedback in an amplifier.
3. Describe one application where positive feedback is intentionally used and explain why.
4. Draw a simple block diagram illustrating a feedback amplifier, labelling the main amplifier and the feedback network.

Lesson Keywords

• Feedback – The process of returning a fraction of the output signal of a system back to its input.
• Positive Feedback – Feedback where the feedback signal is in phase with the input signal, reinforcing it.
• Negative Feedback – Feedback where the feedback signal is 180 degrees out of phase with the input signal, opposing it.
• Amplifier – An electronic device that increases the power or amplitude of a signal.
• Oscillator – An electronic circuit that produces a repetitive electronic signal, often a sine wave or a square wave.
• Stability – The ability of an amplifier to maintain its operating characteristics without unwanted oscillations.
• Bandwidth – The range of frequencies over which an amplifier operates effectively.

Differentiation

For struggling learners: Provide simpler diagrams and focus on the core definitions and basic effects of positive and negative feedback. Use analogies to explain the concepts. Encourage them to ask questions frequently.
For advanced learners: Challenge them to research different feedback configurations (e.g., voltage-series, current-shunt feedback) and their specific effects on input/output impedance. Ask them to explain the conditions for oscillation in positive feedback circuits.

Suggested Lesson Videos

Search on YouTube for: “Feedback in Amplifiers Basic Electronics SS3” or “Positive and Negative Feedback Explained”

Teacher Guide for Using This Lesson Plan

Before the lesson, ensure you have prepared clear and visible charts or diagrams illustrating feedback amplifier block diagrams and examples of positive and negative feedback circuits (e.g., an op-amp with feedback, a basic oscillator). Begin by engaging students with a general concept of feedback before transitioning to electronics. Clearly explain the principles of how feedback works, emphasizing the role of the feedback network. When discussing positive and negative feedback, use the prepared diagrams to visually demonstrate the concepts and their effects. Encourage students to ask questions throughout the explanation. During the discussion phase, prompt students to compare the two types of feedback. Ensure students copy the Board Summary notes accurately into their notebooks after the main teaching points have been covered and evaluated. Pay attention to common misconceptions, such as confusing the effects of positive and negative feedback on gain and stability. For weaker learners, simplify the explanations and provide more guided examples. For faster learners, encourage deeper inquiry into specific applications or mathematical representations of feedback.